In the beverage can industry, it is desirable to have a reliable and economical system and method for inspecting the interior surfaces of certain objects such as beverage cans and containers. Today, manufacturing processes may move, for example, aluminum beverage cans down a conveyor line at speeds of at least 2000 cans per minute. During the manufacturing of the beverage cans, defects may be formed on the interior surface of the cans. It is desirable to reject cans with such defects. Ideally, the interior surface of the cans should be free of physical defects such as puckers and dents. Also, the interior surface should be free of blistered or non-uniform coatings, oil, grease, and debris. The flanges of the cans should be free of knockdowns as well. Typically, these types of flaws occur during the manufacturing of the cans or due to contamination of the cans after manufacturing but before filling with, for example, a beverage.
Machine vision systems are typically used to inspect objects of manufacture such as beverage cans and containers. Machine vision technology allows an image of at least a portion of the object to be sensed and captured. The image may then be processed to determine if any defects are present. Typically, cameras are used to acquire images of the object and a computer or computers are used to process the image. Human vision is very good at analyzing complex objects and scenes but a human is not good at performing repeated tasks over a long period of time without tiring and making mistakes. Machine vision technology allows for sophisticated image acquisition, processing, and analysis of cans and containers on a manufacturing line and provides repeatable performance in real time.
Inspection of cans and containers via machine vision systems presents certain challenges. For example, if the cans or containers are opaque, the vision system must operate on light reflected from the surfaces of the regions to be inspected. Also, the geometry of cans and containers presents various challenges. For example, a typical metal can has a neck that extends upward and radially inward to form an open-topped neck having a smaller radius than the rest of the can. Such a design makes it harder to illuminate and image the entire interior of the can, especially around the surface of the neck of the can.
The field-of-view of cameras used for imaging are often limited and make imaging of the entire interior of an object difficult. For example, wide lens aperture cameras are often used to detect small, unacceptable defects in low light conditions. The depth of focus of a wide lens camera is typically smaller than the height of a beverage can. Therefore, to capture a good image for inspection, the region of the interior surface being imaged using a single camera is, typically, only a portion of the can. A typical beverage can, for example, has a vertical height that does not easily allow a single camera to generate a single image which captures the entire interior of the can, including the rim, neck, sides, and bottom of the can, with sharp focus.
U.S. Pat. No. 5,699,152 to Fedor et al. describes a system and method for inspecting opaque objects, such as metal beverage containers. The system includes a light source for illuminating the interior surface of the container, an ellipsoidal first mirror for forming a first image of an upper interior portion of the container, a first camera for capturing the first image of the upper interior portion of the container, a planar image-splitting second mirror for forming a second image of the flange of the container, a second camera for capturing the second image of the flange, an image combiner for electro-optically combining the first and second images, whereby a resultant composite image corresponding to substantially the entire upper interior surface of the container can be generated and analyzed for defects, a third camera located at a separate location for viewing directly the lower interior portion of the container and capturing a corresponding third image, and a computer means for analyzing the resulting images for defects. Such a system is very complicated, may be difficult to maintain, and can be rather expensive.
It is desirable to figure out how to image the entire interior of beverage cans and containers, as well as the interiors of substantially cylindrical objects in general, using at least a single camera that provides acceptable image quality to minimize cost, complexity, and maintainability of such a vision system.
Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to one of skill in the art, through comparison of such systems and methods with the present invention as set forth in the remainder of the present application with reference to the drawings.